You can unblock someone yourself, in either of two places:
On their profile — open it and click Unblock.
In your settings — go to Settings → Security & Privacy → Blocked Accounts and unblock them from the list.
What changes
They can message you again, find your profile and see your new posts in their feed.
What doesn't come back
Blocking removed your friendship and any follows between you, and unblocking doesn't restore them. If you want to be connected again, follow them or send a friend request — they'll need to accept it.
Need help? Contact support at [email protected]